The present invention relates to a coloured soda-lime.
Soda-lime glass can be clear or coloured, for example, green, grey or blue in transmission.
The expression “soda-lime glass” is used here in the wide sense and relates to any glass which is likely to contain the following principal glass-forming constituents (in percentages by weight):
SiO260 to 75% Na2O10 to 20% CaO0 to 16%K2O0 to 10%MgO0 to 10%Al2O30 to 5% BaO0 to 2% BaO + CaO + MgO10 to 20% K2O + Na2O 10 to 20%. B2O30 to 5% 
In some cases, soda-lime glass can have a total percentage by weight of BaO, CaO and MgO greater than 10%, and even greater than 12%.
This type of glass is very widely used in the field of glazing for automobiles or buildings, for example. It is usually manufactured in the form of a ribbon by the float process. Such a ribbon can be cut into sheets which can then be bent or can undergo treatment to improve their mechanical properties, e.g. a thermal toughening step.
It is generally necessary to relate the optical properties of a glass sheet to a standard illuminant. In the present description, two standard illuminants are used: illuminant C and illuminant A defined by the Commission Internationale de l'Eclairage (C.I.E.). Illuminant C represents average daylight having a colour temperature of 6700 K. This illuminant is especially useful for evaluating the optical properties of glazing intended for buildings. Illuminant A represents the radiation of a Planck radiator with a temperature of about 2856 K. This illuminant describes the light emitted by car headlights and is essentially intended to evaluate the optical properties of glazings intended for automobiles.
The Commission Internationale de l'Eclairage has also published a document entitled “Colorimétrie, Recommandations Officielles de la C.I.E. [Colorimetry and Official Recommendations of the C.I.E.]” (May 1970) which describes a theory in which the calorimetric coordinates for light of each wavelength of the visible spectrum are defined so that they can be represented on a diagram having orthogonal axes x and y, called the C.I.E. trichromaticity plot 1931. This trichromaticity plot shows the locus representative of light of each wavelength (expressed in nanometers) of the visible spectrum. This locus is called the “spectrum locus” and light having coordinates lying on this spectrum locus is said to have 100% excitation purity for the appropriate wavelength. The spectrum locus is closed by a line called the purple boundary which connects the points of the spectrum locus, the coordinates of which correspond to wavelengths of 380 nm (violet) and 780 nm (red). The area lying between the spectrum locus and the purple boundary is that available for the trichromaticity coordinates of any visible light. The coordinates of the light emitted by illuminant C, for example, correspond to x=0.3101 and y=0.3162. This point C is regarded as representing white light, and consequently has an excitation purity equal to zero for any wavelength. Lines may be drawn from point C to the spectrum locus at any desired wavelength and any point lying on these lines may be defined not only by its x and y coordinates, but also as a function of the wavelength corresponding to the line on which it lies and of its distance from point C relative to the total length of the wavelength line. Consequently, the colour of the light transmitted by a coloured glass sheet may be described by its dominant wavelength (λD) and its excitation purity (P) expressed as a percentage.
The C.I.E. coordinates of light transmitted by a coloured glass sheet will depend not only on the composition of the glass but also on its thickness. In the present description, as in the claims, all the values of the excitation purity P and the dominant wavelength λD of the transmitted light are calculated from the spectral specific internal transmissions (SITλ) of a glass sheet 5 mm in thickness with illuminant C from a solid viewing angle of 2°. The spectral specific internal transmission of a glass sheet is governed solely by the absorption of the glass and can be expressed by the Beer-Lambert law:SITλ=e−E.Aλ                where Aλ is the absorption coefficient (in cm−1) of the glass at the wavelength in question and E the thickness (in cm) of the glass. In a first approximation, SITλ may also be represented by the formula:(I3+R2)/(I1−R1)        where I1 is the intensity of the incident visible light on a first face of the glass sheet, R1 is the intensity of the visible light reflected by this face, I3 is the intensity of the visible light transmitted from the second face of the glass sheet and R2 is the intensity of the visible light reflected by this second face towards the interior of the sheet.        
The following are also used in the following description and the claims:                for illuminant A, the total light transmission (TLA) measured for a thickness of 4 mm (TLA4) from a solid viewing angle of 2°. This total transmission is the result of the integration between the 380 and 780 nm wavelengths of the expression: Σ Tλ.Eλ.Sλ/Σ Eλ.Sλ in which Tλ is the transmission at wavelength λ, Eλ is the spectral distribution of illuminant A and Sλ is the sensitivity of the normal human eye as a function of wavelength λ;        the total energy transmission (TE) measured for a thickness of 4 mm (TE4). This total transmission is the result of the integration between the 300 and 2500 nm wavelengths of the expression: Σ Tλ.Eλ/ΣEλ. The energy distribution Eλ is the spectral energy distribution of the sun at 30° above the horizon with an air mass equal to 2 and an inclination of the glazing of 60° relative to the horizontal. This distribution, called “Moon distribution”, is defined in the standard ISO 9050;        the selectivity (SE) measured as the ratio of the total light transmission for illuminant A to the total energy transmission (TLA/TE);        the total transmission in the ultraviolet, measured for a thickness of 4 mm (TUV4). This total transmission is the result of the integration between 280 and 380 nm of the expression: Σ Tλ.Uλ/Σ Uλ in which Uλ is the spectral distribution of the ultraviolet radiation that has passed through the atmosphere, defined in the standard DIN 67507;        the Fe2+/total Fe ratio, sometimes called the redox ratio, which represents the value of the ratio of weight of atoms of Fe+2 to the total weight of iron atoms present in the glass and is obtained by the formula:Fe2+/total Fe=[24.4495×log(92/τ1050)]/τFe2O3         where τ1050 represents the specific internal transmission of the 5 mm thick glass at the 1050 nm wavelength, tFe2O3 represents the total iron content expressed in the form of oxide Fe2O3 and measured by X-ray fluorescence.        
Coloured glass can be used in architectural applications and as glazing for railway carriages and motor vehicles. In architectural applications, glass sheets 4 to 6 m in thickness are generally used, while in the automotive field thicknesses of 1 to 5 mm are normally employed, particularly for the production of monolithic glazing, and thicknesses of between 1 and 3 mm in the case of laminated glazing, especially for windscreens, two glass sheets of this thickness then being bonded together by means of an interlayer film, generally made of polyvinyl butyral (PVB).